Specular gloss of paper and paperboard at 20 degrees

Transcription

1 T 653 om-98 OFFICIAL TEST METHOD 1980 WITHDRAWN 1985 REVISED AND REINSTATED 1990 PROVISIONAL METHOD 1990 OFFICIAL METHOD TAPPI The information and data contained in this document were prepared by a technical committee of the Association. The committee and the Association assume no liability or responsibility in connection with the use of such information or data, including but not limited to any liability or responsibility under patent, copyright, or trade secret laws. The user is responsible for determining that this document is the most recent edition published. Specular gloss of paper and paperboard at 20 degrees 1. Scope 1.1 This method is for the measurement of the specular gloss of high-gloss papers at 20 (70 from the plane of the paper). 1.2 This method is suitable for high-gloss coated, cast-coated, lacquered, highly varnished or waxed papers, and high-gloss ink films. For the measurement of low- to moderate-gloss coated and uncoated papers and low- to moderate-gloss ink films, TAPPI T 480 Specular Gloss of Paper and Paperboard at 75 Degrees is preferred. Neither method is a measure of image reflecting quality. 1.3 This method will provide different values for 20º gloss than ASTM Standard D523 due to differences in optical geometry. 2. Summary This method involves the measurement of reflectance using a scale on which a polished black glass with a refractive index of measures 100 units. The angles of incidence and reflectance are each 20, the receptor or viewing window is round and its diameter subtends an angle of 5 with respect to an apex at the center of the illuminated area in the plane of a specimen opening. These geometric conditions establish a gloss scale which provides optimum discrimination for high gloss papers. (1) 3. Significance This method gives good correlation with visual gloss or shininess provided the sample is flat. The degree of correlation decreases as the sample deviates from being flat. 4. Definition Specular gloss - ratio of flux reflected in specular direction to incident flux for specified source and receptor apertures (usually measured relative to a standard of specified index of refraction). Approved by the Optical Properties Committee of the Process and Product Quality Division TAPPI

2 T 653 om-98 Specular gloss of paper and paperboard at 20 degrees / 2 5. Apparatus Gloss meter, which has the general arrangement and significant dimensions of the optical system shown in Figure 1. The instrument consists of a source of light, a lens giving a converging beam of rays incident on the test specimen, a suction plate to hold the specimen flat, a receptor window, a receiving lens, a filter to modify the spectral distribution of the light, and a photodetector to receive and measure the rays reflected by the test specimen. These components are combined in a light-tight housing that is matte black inside and is structurally and optically stable throughout a normal range of operating temperatures 23.0 ± 5ºC (73.4 ± 9ºF). The instrument must also meet the geometric, photometric and spectral conditions described as follows: Geometric conditions - The axis of the incident beam is 20 ± 0.15 from a line perpendicular to the plane of the test surface. The receptor window, the lens adjacent to this window, and the photodetector are centered about an axis which is the mirror image of the axis of the incident beam. Thus, an image of the source field stop shall be formed in the center of the receptor window when a front-surface mirror is placed in the specimen position. The diameter of this image shall subtend angles at the center of the illuminated area in the specimen plane of no more than 2.5 nor less than 1.0. The receptor window is round in shape and is of such a diameter that it collects a solid angle of light rays of 5 ± 0.5 with respect to the center of the illuminated area in the plane of the specimen opening (Figure 1). Fig Glossmeter optical system. Diaphragms and light shields are provided to keep all light from reaching the photodetector except that light present in the beams described above without any vignetting or interception by stops or diaphragms of rays projected in the directions specified. The interior of the instrument is finished in matte black, and the lamp, lenses, photodetector and interior surfaces are to be kept clean and free of dust to minimize stray light Photometric conditions - Any combination of photodetector and indicating device may be used provided it gives a numerical indication in direct proportion to light flux passing through the receptor window over the entire scale to within ± 0.2% of full scale: i.e., ± 0.2 scale division for a scale comprised of 100 divisions. The photometric linearity 1 Names of suppliers of testing equipment and materials for this method may be found on the Test Equipment Suppliers list in the bound set of TAPPI Test Methods, or may be available from the TAPPI Technical Operations Department.

3 3 / Specular gloss of paper and paperboard at 20 degrees T 653 om-98 may be established by using the procedure described in TAPPI T 1217 Photometric Linearity of Optical Properties Instruments Spectral conditions The source and photodetector are spectrally corrected by means of a filter (or filters) so as to give the combination a spectral response duplicating the CIE luminosity function. This response function is referred to as Y a. 5.2 Vacuum plate specimen holder - The surface of the suction plate is matte black in color with a reflectance of less than 1% and so smooth that the paper is not visibly marked when a vacuum is applied. The surface of the suction plate is flat to within 25 micrometers (0.001 in.). Vacuum can be applied to the surface of the paper either by using a porous plate or by supplying the vacuum through a ring indentation surrounding the test area. If the porous plate is used, the pores can be expected to plug up (because of the removal of small suspended particles in the air being drawn through the plate) and frequent backwashing of the plate becomes necessary. It is important with either type of device that the vacuum does not distort the surface of the paper during measurement. A thin sheet of high gloss polyester film such as Mylar (0.003 in. thick) when held by the suction plate will not produce an image visibly different in size at the photodetector than the image formed by a polished black glass standard. (For heavy weight materials such as board or paperboard, a 1 kg weight may be used as a backing material.) 6. Reference standards A high-gloss standard of polished black glass is required and at least one intermediate-gloss standard of stable ceramic material is recommended. 6.1 High-gloss standard black glass - The gloss value of a plane polished black glass standard, having an index of refraction of 1.540, is defined to be 100 units. Whenever a standard not having an index of refraction of is to be used, its specular reflection (R ) may be computed from its refractive index using the Fresnel equation: s where: I = angle of incidence (20 deg), r = angle of refraction and n = refractive index. Example: A black-glass working standard having a refractive index of will have a specular reflectance of Similarly, the specular reflectance of the 100-unit primary standard black glass (n = 1.540) is Thus, the value of the working standard on the present scale would be ( / ) x 100 = Alternatively, when the standard has an index differing from 1.540, the appropriate instrumental gloss value can be obtained by adding or subtracting 0.29 gloss units for each in index that the standard is higher or lower than Intermediate standard of satin or matte-finish ceramic tile carefully calibrated in terms of a black-glass standard on an instrument known to comply with the established requirement of this method. (For any test, it is advantageous to have an intermediate standard of about the same gloss as the specimens to be measured.) 7. Sampling and test specimens 7.1 From each test unit obtained in accordance with TAPPI T 400, Sampling & Accepting a Single Lot of Paper, Paperboard, Containerboard or Related Product, cut at least ten test specimens free from folds or wrinkles or other blemishes and of sufficient size (50-75mm square) to cover completely the specimen opening of the instrument

4 T 653 om-98 Specular gloss of paper and paperboard at 20 degrees / 4 with an adequate overlap. Keep the specimens clean and do not touch the area to be tested. Condition and test paper in an atmosphere of 50% + 5.0% % RH and 23.0 ± 5ºC (73.4 ± 9ºF). NOTE 1: The exposure of paper to relative humidities of about 65% or above progressively and irreversibly decreases the gloss (2). NOTE 2: Specimen sources of discrepancy: 1. Specimen nonflatness is potentially a serious source of discrepancy. The mounting device must hold each sheet flat. If a vacuum plate is used, it must not be so coarse that it wrinkles the paper surfaces. 2. Specimen nonuniformity. Some surface variation in gloss from area to area is normal. In addition, surface damage by abrasion, blocking, scratching, etc. may importantly change the gloss of certain areas. 3. Differences in opacity, backing, and background color introduce small discrepancies which correspond to the differences in diffuse reflectance. A white printed area may measure two or three units higher than an identical surface area printed black. Readings for papers of low opacity will similarly change with change from black to white backing. This change may be almost two units for transparent waxed paper or glassine, but is negligible for coated opaque papers. 8. Procedure 8.1 Operate the gloss meter in accordance with the manufacturer s instructions with regard to warm up time, etc. 8.2 Clean the black, high-gloss standard and intermediate standard carefully in accordance with the manufacturer s instructions. The cleaning and drying procedure is particularly critical for the intermediate standards. The following cleaning and drying procedure has been found to be adequate: Using a mild (non-fluorescent) detergent solution (such as one tablespoon detergent in one gallon water), brush the solution on the surface of the standard with a soft nylon brush followed by a forced rinse with hot water (approximately 150 F). Immediately blot (do not rub) the standard dry with a clean, lint-free, paper towel or tissue An alternative procedure for cleaning the high-gloss standard is to place two drops of glass cleaner with ammonia on the polished surface, then rub the solution across the polished surface with a clean index finger. Wipe thoroughly dry with a lint-free paper towel or tissue. To clean the intermediate standards, place two drops of glass cleaner with ammonia on the surface to be measured and rub the solution across the surface with a clean index finger. Place the standard face-up on a table or countertop and blot (do not rub) dry with a clean, lint-free, paper towel or tissue. Press hard when blotting to remove moisture from the pores and crevices. It is important not to rub the paper towel or tissue across the face of the intermediate standard. 8.3 With the instrument turned on and warmed up, the instrument should read 0 with the specimen aperture covered with a black velvet-lined cavity to prevent external light from entering the receptor window. A reading other than 0 suggests that stray light is present in the optical system or the electronics are not properly zeroed. 8.4 Place the clean, black, high-gloss standard in the specimen position and adjust the instrument to give the assigned value of gloss for the standard. Place the intermediate standard in the specimen position and observe the reading. Correct readings on the black, high-gloss standard and intermediate standards suggest that the instrument is in approximate, but not necessarily in exact, conformance with the apparatus specification. If readings differ by more than one gloss unit from assigned values, the instrument should be checked for conformance to the geometric, spectral and photometric requirements. 8.5 Following the calibration check, measure at least five specimens or areas with the light path parallel to the machine direction. Measure the same number of areas with the light path parallel to the cross-machine direction. Repeat for the other side of the sample if desired. NOTE 3: If the gloss value in the MD values vary by more than 1.0 unit when the sample is rotated 180º, half of the MD measurements must be made in each orientation. 9. Report Report the average of the MD and CD measurements as the TAPPI Gloss value for the side tested to the nearest unit. The minimum and maximum values and other data regarding the population of test values may also be reported. NOTE 4: If your purposes require MD or CD values, at least 10 measurements are required in each direction of interest. These values must be reported as MD or CD, as appropriate.

5 5 / Specular gloss of paper and paperboard at 20 degrees T 653 om Precision 10.1 The following estimates of precision are based on an interlaboratory trial involving six samples ranging from 6 to 36 gloss units. Testing was conducted in twelve laboratories; ten test results were made for each sample. The terms repeatability and reproducibility are used as defined in TAPPI T 1206 Precision Statement for Test Methods. A summary of the results follows: Number Number Mean r R of labs Mean r R of labs Repeatability and reproducibility are estimates of the maximum difference (at 95% confidence) which should be expected when comparing measurements. These estimates may not be valid for all specimens, procedures and conditions The results of interlaboratory testing for this precision statement indicate that material variability is a major determinant to the precision of the method. 11. Keywords Paper, Paperboard, Gloss 12. Additional information 12.1 Effective date of issue: January 22, This method was first published in 1958 as a suggested method. It was revised in 1961, became an official standard in 1970 and was subsequently revised in 1976 and It was withdrawn in 1985 along with a series of Wax Testing Methods. It was reinstated again in 1990 and the title was changed from Specular Gloss of Waxed Paper (20 ) to Specular Gloss of Paper and Paperboard at Related methods: TAPPI T 480, ASTM D ASTM Standard D 523 will provide different values for 20 gloss than this method due to differences in optical geometry (primarily the difference in solid cone angles of incident and receiving optics). Literature cited 1. Hunter, R. S. and Lofland, C. A., A Gloss Test for Waxed Paper, Tappi 39 (12): (Dec. 1956). 2. The Institute of Paper Chemistry, Instrumentation Studies XII, Effect of Relative Humidity on Physical Properties with Respect to the Hysteresis Effect in Changes from One Humidity to Another, Paper Trade J. 104 (15): 45 (April 15, 1937). References Harrison, V. G. W., The Definition and Measurement of Gloss, Cambridge, England, W. Heffer and Sons, Ltd., 145 pages. Wink, W. A., Delevanti, C. H. Jr., and Van den Akker, J. A., Instrumentation Studies, LXXVII, Goniophotometric Study of High-Gloss Papers, Tappi 36 (12): 163A (1953). Hunter, R. S., Method of Evaluating the Gloss of Ink Films, Tappi 46 (7): 162A (1963). Budde, W., The Calibration of Gloss Reference Standards, Metrologia 16: 89 (1980). Budde, W., and Dodd, C. X., Stability Problems in Gloss Measurements, J. Coatings Technology 52: 44 (1980). Budde, W., Polarization Effects in Gloss Measurements, Applied Optics 16, 2252 (1979). Your comments and suggestions on this procedure are earnestly requested and should be sent to the TAPPI Technical Operations Manager.